This invention relates to a coil and to a capacitor for a radio frequency (RF) security element with a capacitor and with a spiral-wound coil having several windings; and further to a method for production of a coil and of a capacitor for an RF security element.
RF security elements are comprised of a resonant circuit with a coil and a capacitor. There is a vast variety of RF security element designs, such as those embodied as self-adhesive labels, as tags or so-called xe2x80x9chard tags.xe2x80x9d These hard tags are made up of a resonant circuit enclosed in a sturdy casing that is in turn so firmly attached to the article to be protected that a potential shoplifter can not remove it. If the article is properly paid for, a hard tag of this nature is detached from the article, and the buyer can leave the store unhindered. Surveillance zones containing an alternating magnetic field are usually set up in the exit area of stores. If an article with a hard tag still attached to it reaches the surveillance zone, the resonant circuit of the hard tag is excited by the alternating magnetic field to emit a characteristic recognition signal. The alarm is triggered as soon as the surveillance system registers this recognition signal.
For the highest possible detection rate the resonant frequency setting of the resonant circuit in the hard tag must be very precise. On the other hand, hard tags are auxiliary devices that should be available at the lowest possible cost.
The object of the invention is to provide an RF security element which has a resonant frequency that can be set with low tolerances, and which can be produced at low cost.
This object is achieved in accordance with the invention by a coil for an RF security element with a capacitor and with a spiral-wound coil having several windings, in which windings of straight wire and windings of undulated wire are provided, preferably alternating with one another. The mean distance between adjacent windings is very large due to the preferable alternation of one winding with straight wire and one with undulated wire. Thus, the coil has a low capacitance and is of correspondingly high quality factor or Q factor. Furthermore, the coil according to the invention is easy and inexpensive to produce, since fewer demands are made on the wire used. This is why, for example, it is not necessary to use the PVC or silk insulation ordinarily used for high-quality coils. This substantially lowers the cost of the wire.
In a further development of the invention the wave valleys and peaks of the undulated windings touch the adjacent windings of straight wire or are arranged in their direct vicinity, so that the space required for the coil stays as small as possible in spite of a large mean distance between neighboring windings of the coil. Furthermore, spatial displacements of the windings relative to one another are prevented, for which reason the electrical properties of the resonant circuit are also stable.
In other embodiments the undulated wire is provided in the shape of half-waves of a triangular, sine half-wave or semicircular shape. This permits the production of the undulated wire to be as simple as possible.
In one embodiment of the invention, each winding of straight wire and each winding of undulated wire encloses an angle of 2xcfx80. Therefore, the coil according to the invention is of especially low capacitance.
A variant of the invention provides for the coil to be produced from baked-enamel wire, so that the material costs for production of the coil can be drastically reduced as opposed to the known spiral-wound coil with PVC or silk insulation.
The object given above is also achieved by a capacitor for an RF security element with a spiral-wound coil having several windings and with a capacitor, in which the capacitor comprises a first piece of wire and a second piece of wire, the first and the second wire pieces being intertwined. In this manner, expensive commercially available capacitors are replaced with two pieces of wire.
In a further development of the invention the first piece of wire or the second piece of wire is wound about the respective other piece of wire. This simplifies the twisting and lowers the production costs.
In another embodiment the first piece of wire comprises the one end of the coil wire and the second piece of wire comprises the other end of the coil wire. Thus, the entire resonant circuit can be produced from one piece of wire, and it becomes unnecessary to join together the coil and the capacitor. This further reduces the production costs.
In a further development of the invention the capacitor is made of baked-enamel wire. This permits the material costs for the capacitor according to the invention to be kept low.
The object given above is also achieved by an RF security element with a spiral-wound coil having several windings and with a capacitor, in which the coil is a coil according to one or more of the disclosed embodiments of the invention or equivalents thereof, and in which the capacitor is a capacitor according to one or more of the disclosed embodiments of the invention or equivalents thereof. This combines within one RF security element the advantages not only of the coil according to the invention, but also of the capacitor according to the invention.
The object given above is further achieved by a method for production of a spiral-wound coil of an RF security element in which a winding of straight wire and a winding of undulated wire, preferably alternating, are wound about a winding spindle. A coil according to the invention can be produced by this method, which results in the advantages given above.
In a further development of the invention, the undulated wire is produced by passing the originally straight wire through at least two mating gears. In this simple way, continuous production of undulated wire is possible. Another embodiment of the method according to the invention provides for the coil to be produced in accordance with one or more of the methods disclosed herein or equivalents thereof. In this way the above-given advantages of the method according to the invention for producing a coil also come to bear.